Gas phase polymerization is the predominant reactor technology used to produce olefin plastic resins. The catalysts are contained in solid substrate particles from which the polymer chains grow. The particles are fluidized in a fluidized bed by a gas stream containing the monomers.
Gas phase fluidized bed reactors are used to produce linear low density polyethylene (LLDPE) resins, which are the largest and fastest growing segment of the polymer market. LLDPE resins are copolymers of ethylene and from between about 3% to about 10% by weight an alpha-olefin comonomer. Suitable alpha olefin comonomers for LLDPE resins include propylene; butene 1; 4-methyl pentene-1; hexene-1; and octene-1. Butene-1 and hexene-1 are used in highest volume. LLDPE resins are characterized and defined by their density which is in the range of between about 0.91 and 0.94 g/cc. Gas phase reactors are also used to produce the ethylene homopolymers, high density polyethylene (HDPE), which has a density in the range of between about 0.94 and 0.98 g/cc.
Over 50% of LLDPE production is used to make films and stretch wrap packaging. LLDPE resins are also used in wire and cable insulation and jacketing, rotomolding applications, pipe and conduit, and blow molding and extrusion coating applications.
In 1992 about 3,500,000 tons of LLDPE resins were produced in the U.S. using fluidized bed gas phase technology. A limited volume of ethylene-propylene elastomers for specialized applications are produced using fluidized bed technology and Ziegler-Natta catalysts.
The polymerization reactions are exothermic. The heat of reaction is absorbed by the reactor gas as sensible heat which increases the temperature as it flows up through the reactor. The heat of reaction is dissipated by cooling the reactor gas stream in heat exchangers prior to recycling the gas to the reactor inlet.
In recent designs the heat removal capacity of the reactor is significantly increased by adding catalytically inert condensible liquids such as penlane or hexane to the reactor feed gas stream. This development flies in the face of long-standing conventional wisdom which taught that stable fluidization could not be sustained if the reactor gas contained more than a minor amount of liquid. This is an important development because, in general, heat removal capacity is the major bottleneck that limits production capacity of the reaction section. The liquids vaporize in the reaction zone, thus increasing the heat removal capacity in the reactor by absorbing heat of vaporization (latent heat) in addition to the sensible heat increase of the reactor gas. The inert condensibles are condensed in the recycle heat exchangers and the liquid is recycled to the reactor, either entrained in the recycle gas stream or separated and recycled to the reactor as a separate liquid stream.
Polymerization reaction temperature is typically in the range from about 85.degree. F. to about 220.degree. F. and pressure from about 150 to about 350 psig. Ethylene partial pressure in the reactor gas phase is typically in range of from about 20 to about 150 psia and comonomer partial pressure is typically in the range of from about 5 to 50 psia. The gas phase typically includes nitrogen (or other inert gas) to provide sufficient linear velocity for smooth fluidization, and to makeup the difference between the partial pressure of the monomers and the total pressure required in the gas phase. Nitrogen also is a sensible heat sink which absorbs heat of reaction with increase of temperature as the reactor gas flows up through the reactor. A small amount of hydrogen is added to the gas phase to control the molecular weight (melt index) of the polymer via chain transfer reactions.
To maintain the fluidized bed at a constant height, polymer reactor product in the form of a fine granular powder is continually discharged from the reactor entrained in a stream of reactor gas at a rate equal to the rate of polymer formation in the reactor. The reactor product is discharged into a product discharge tank in which most of the reactor gas is separated from the product polymer powder and the separated reactor gas is recycled back to the reactor. Typically, the reactor product powder is conveyed out of the discharge tank by a stream of nitrogen or other inert gas into a product purge tank. The reactor product flowing from the product discharge tank unavoidably carries entrained reactor gas that contains unreacted monomers and solvents as well as the purge nitrogen. The product polymer also contains monomers and solvent that are dissolved and physically adsorbed in the polymer.
Entrained and adsorbed reactor gas must be removed from the product polymer powder before the product is conveyed on to storage or further processing to preclude forming explosive gas mixtures. Otherwise, the monomers and other hydrocarbons will diffuse out of the product polymer powder in downstream vessels which contain air. The requisite purging is accomplished in the product purge tank by blowing a stream of nitrogen countercurrently up from the bottom of the purge tank through the polymer. This flushes out entrained reactor gas and strips and desorbs dissolved hydrocarbons out of the product powder.
Typically, the vent gas stream is disposed of by burning it in a flare stack. Following are typical stream rates for the vent gas stream from the product purge tank in an LLDPE unit producing 22,000 lbs. per hour of an LLDPE butene-1 copolymer using hexane as the cocatalyst/activator solvent:
______________________________________ Component Flow Rate (lb./hr) ______________________________________ Ethylene 265 Butene-1 238 Hexane 25 Nitrogen 610 Ethane 27 Hydrogen 1 C4's 15 ______________________________________
The comonomers lost in product vent stream represent a substantial economic loss to polyolefin producers. Also, there is the continually rising cost of disposing of the vent gas to meet air quality protection codes. Typically, from about one to about two percent of the ethylene fed to the process is lost in the vent gas stream. Comonomer losses are much higher ranging from about 10% for butene-1 to about 50% for hexene-1. The fraction of comonomer that is lost in the vent gas stream increases with molecular weight of the comonomer because the solubility of comonomer in the product polyolefin increases with the molecular weight of the comonomer. Monomer and comonomer vent losses add between about one to two cents per pound to the cost of making polyolefins which is a substantial sum considering that worldwide production of gas phase polyolefins is approaching 20 million tons per year. In addition, the vent gas also contains significant amounts of hydrocarbon solvents such as hexane which also have value.
Thermal cracking of hydrocarbon feedstocks is the primary production route to ethylene and other olefin monomers. The high cost of olefins and the large capital investments for new thermal cracking plants logically should motivate olefin polymerization operators to recover ethylene and comonomer olefins from polyolefin reactor vent streams before purchasing monomers. Why don't polyolefin operators recover ethylene and other valuable components from vent gas streams? The answer is that the only technology currently on the market to recover monomers from vent gas is cryogenic technology and with the cryogenic processes currently available, it is cheaper for operators to buy or produce makeup monomers than to recover the monomers from vent gas. Moreover, cryogenic processes are not flexible and do not adapt to changes in feed composition and feed gas flow rates that occur in polyolefin plants. So that although monomer recovery from polyolefin reactor vent gases is technically feasible, it is impractical using currently available commercial cryogenic recovery processes.
For the foregoing reasons, there is need for a flexible and cost effective process for recovering ethylene, other alpha olefin comonomers and hydrocarbon solvents from vent gases emitted from gas phase olefin polymerization plants.
The solution lies in Mehra processes which are absorption processes that utilize a physical absorption solvent to separate and recover hydrogen, nitrogen, methane, ethylene and other valuable hydrocarbons from mixed hydrocarbon streams. Mehra technology has been applied to recover ethylene, hydrogen and methane from refinery and petrochemical off-gas streams and to reject nitrogen from natural gas. Generally, Mehra processes compete with cryogenic processes in these applications. Depending on the application, Mehra specifies absorption solvents that are selected from preferred groups and process designs which optimally synergize solvent with process. Among the preferred Mehra solvents are C4 to C10 hydrocarbons including paraffins, naphthenes and aromatics. Mehra technology is described in U.S. Pat. Nos. 4,832,718, 4,740,222, 5,019,143, 5,220,097 and 5,326,929, which are incorporated herein by reference.
In general, Mehra processes operate at a higher temperature than cryogenic processes which provides advantages over cryogenic processes: 1) Exotic cryogenic construction materials required to withstand cryogenic temperatures are not required in Mehra processes; 2) Feed purification specifications are more relaxed; 3) Cryogenic processes are intensively heat integrated to reduce energy consumption whereas Mehra processes are not. Accordingly, Mehra processes are more flexible and adaptable to changes. Process conditions can be changed quickly "on-line" with no adverse impact on operability and without equipment modifications to alter product stream compositions or maintain product composition should feed composition change.